Accurate measurement of IOP, pressure inside the eye, is extremely important in making the diagnosis of pressure related eye diseases, particularly glaucoma, and in management or making treatment plans such as choosing medications, laser treatment or surgery to control these serious eye diseases.
Applanation tonometry (AT) has long been the standard for clinical measurement of LOP and has generally been accepted to reflect true LOP by generations of ophthalmologists. In fact most ophthalmic literature assumes AT to be LOP. Goldmann's approach is represented by equations based on a modification of principles described by Imbert and by Maklokoff in 1885 and Fick in 1887. Imbert, A., “Theorie sur ophthalmotonometre,” Arch Ophthalmol (Paris) 5:3 58–363 (1885); Fick, A., “Ueber Messung des Druckes im Auge” Aech fur Die Gesammte Physiologie des Menschen & der Thiere 42:86–90 (1888). Imbert and Fick analyze the relationship of forces acting on an eye model assumed to be dry and a perfect sphere with an infinitely thin wall. However, the cornea satisfies none of the above conditions.
The current widely used method of measuring IOP by applanation tonometry was developed by Goldmann based on studies on cadaver eyes in 1957. Goldmann discussed the limitations of applying the Imbert-Fick's principle to the cornea. Goldmann, V. H. Schmidt T., “Uber applanationstonometrie,” Opthalmologica 134:221–242 (1957). The wet surface of the eye creates surface tension (S), and the thickness of the wall creates a counterforce (E) against the force (W) applied on the sphere surface.
Imbert-Fick's Principle States:P+E=W/A−S  (1)In which:    P=IOP as measured by the tonometer    E=modulus of elasticity of corneal deformation, corneal thickness being a major factor    W=the force acting on the tonometer tip    A=area of contact between the tonometer tip and flattened cornea    S=the attractive force of surface tension
By measuring various variables involved, an applanation prism was designed and calibrated with a diameter of 3.06 mm assuming the corneal thickness to be 0.5 mm, thus canceling surface tension (5) and effect of the thickness of the cornea (E), simplifying the equation toP=WIA   (2)Using known or measured values of A and W, IOP is determined.
Goldmann measured the thickness of a handful of eyes in Switzerland and assumed the corneal thickness to be a constant value of half a mm. It was necessary for him to reduce variables in Imbert-Fick formula to devise his instrument. He stated in his writing that theoretically the variation in corneal thickness will affect the reading of IOP, but there is no indication he was aware of the magnitude of error caused by corneal thickness.
Goldmann was also aware of the effect of corneal curvature (K) on applanation reading. He devised the prism to be rotatable to get applanation readings in the steepest and flattest meridian of the cornea. In highly astigmatic corneas, the steeper meridian yields a higher reading. In astigmatic corneas, the area of the oval surface applanated has to be compared to the ideal round surface.
Ehlers et al of Denmark in 1975 studied the relationship between central corneal thickness (CCT) and AT in rabbits and 29 human eyes. Ehlers, H. et al., “Applanation tonometry and central corneal thickness,” Acta Ophthalmol 53:34–43, (1975). Ehlers et al. measured CCT by optical means, cannulated human eyes in vivo and compared the defined IOP and AT measured with a Perkins or Draeger hand held applanation tonometer calibrated against standard Goldmann tonometer. They found statistically significant correlation between CCT and error of AT(ΔP). Having seen a linear relationship between the two variables in preliminary study, they calculated the intermediate pressure level from ΔP 10 and ΔP 30 by linear interpolation. They offered a correction table to obtain IOP from CCT and AT. They eliminated eyes with astigmatism greater than 1.5 D to avoid the errors caused by astigmatic eyes. They saw a linear relation between CCT and K in rabbit eyes but not in 29 human eyes they studied. They predicted that study in larger human sample might confirm the similar relation in humans. Over a span of 140 micron of difference in CCT, error of AT (ΔP) ranged 8.7 mmHg (−4.5 to +4.2) at an AT level of 10 mmHg; it ranged 9.3 mmHg (−4.6 to +4.7) at an AT level of 15 mmHg; it ranged 9.9 mmHg (−4.7 to 5.2) at an AT level of 20 mmHg; it ranged 10.5 mmHg (−4.8 to +5.7) at an AT level of 25 mmHg and 11.1 mmHg (−4.9 to +6.2) at an AT level of 30 mmHg.
In 1995, Argus of Indiana studied CCT of 36 patients with ocular hypertension (OHT), 29 control subjects and 31 patients with glaucoma. Argus, Wash., “Ocular hypertension and central corneal thickness,” Ophthalmol., 102:1810–1812 (1995). CCT of OHT was 610 micron, which was significantly greater than glaucoma (557 micron) and control (567 micron). He concluded that corneal pachymetry to be clinically helpful in estimating IOP, determining the risk of visual loss and establishing a target pressure. Using ultrasonic pachymetry, he found the average corneal thickness in 96 eyes to be 567 microns.
Stodtmeister of Germany in 1998 measured CCT in 579 patients using ultrasonic pachymetry. Stodtmeister, R., “Applanation tonometry and correction according to cornea thickness,” Acta Ophthalmol Scand 76: 319–324 (1998). From the thickness obtained, the correction values for IOP were calculated. Correction values of +/−2 mmHg and above were found in 50% of the patients examined, correction value of +/−3 mmHg and above in 25%+patients, and correction value +/−4 mmHg and more in 20% of patients. He used the normal corneal thickness value of 578 micron and a linear correction formula of P=A+(578−T)/14 derived from Ehlers results, proposed by Argus.
However, a need still exists for convenient and more accurate methods and systems for obtaining corrected interocular pressure values.